Experiments will be performed on fetal sheep of known gestational ages. Indwelling fetal catheters, flow sensors and amniotic and allantoic fluid catheters will be placed during sterile surgeries under general anesthesia. Hypotheses to be tested include: (l) Urine production, although variable, detracts from, rather than contributes to the control of amniotic fluid volume. (2) Neither lung fluid production, nor drinking of amniotic fluid are necessary for an adequate homeostatic response to abnormal production of amniotic fluid. (3) Fetal swallowing may contribute to amniotic fluid volume regulation even if not necessary in sheep. (4) Reabsorption of amniotic fluid is largely insensitive to electrolyte load. (5) Neither the crystalloid osmotic gradient nor the oncotic gradient between amniotic fluid and fetal plasma are involved in volume regulation. (6) The quantity that is being regulated is amniotic fluid volume, rather than intrauterine volume. (7) The absorptive mechanism permits passage of large quantities of macromolecules, such as plasma albumin, even in the absence of drinking.